1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge.
2. Discussion of the Background
Among the image bearing members applied to photocopiers, laser printers, etc., inorganic image bearing members formed of selenium, zinc oxide, cadmium sulfide, etc. used to be the mainstream. However, organic image bearing members (photoconductors: OPC) are now dominant over the inorganic image bearing members since the organic image bearing members are advantageous in terms of the burden on the global environment, cost reduction, free latitude of designing, etc. The production ratio of the organic image bearing member is almost 100% of the total production of the image bearing member. Such organic image bearing members are required to shift from a disposal supply product to a mechanical part upon the ground swell of the global environmental protection.
Various kinds of improvements on the durability of the organic image bearing member have been attempted so far. Among these, a technology of forming a cross-linked resin layer {for example, unexamined published Japanese patent application No. (hereinafter referred to as JOP) 2000-66424} or a sol-gel cured layer (for example, JOP 2000-171990) on the surface of an image bearing member especially shows promise. The former technology is advantageous in that cracking hardly occurs when a charge transport component is blended, which leads to improvement on the yield ratio. Especially, radical polymerizable acrylic resins are suitable to manufacture a strong and highly sensitive image bearing member. Since the layer of the two technologies employing the cross-linked structure is formed by multiple chemical linkages, the surface is not immediately abraded when part of the chemical linkages is severed under the stress on the layer.
On the other hand, the toner for use in electrophotography is suitable in terms of ecologies relating to manufacturing and improvement on image quality. Therefore, using a polymerization toner (spherical toner) is popular these days.
This polymerization toner (spherical toner) is not angular but round and manufactured by a chemical manufacturing method such as a suspension polymerization method, an emulsification agglomeration method, an esterification elongation polymerization method, a dissolution suspension method, etc. The form of the polymerization toner depends on the manufacturing method. The polymerization toner for use in an image forming apparatus is slightly irregular shaped in comparison with a sphere. The polymerization toner has characteristics such as an average circularity of 0.95 to 0.99 and shape factors SF-1 and SF-2 of from 110 to 140. A sphere has an average circularity of 1.0 and shape factors SF-1 and SF-2 of 100.
Since the polymerization toner has a uniform shape, the amount of charge held thereby is relatively uniform. In addition, waxes (5 to 10%), etc. can be easily internally added to the polymerization toner. Therefore, since the polymerization toner hardly strays out of a latent electrostatic image, the polymerization toner has a good development property and transfer efficiency, and is suitable for producing sharp images having a high definition and excellent graduation. In addition, oil is unnecessary for image transfer. However, this kind of toner has bad cleaning property and the addition amount of external additives inevitably increases due to the employment of the oil-free system. This may cause filming having a killifish form on an image bearing member. A number of studies have been made to deal with this problem and a great number of technologies therefor are described in patent documents.
Generally, image bearing members using a polymerization toner are desired to have and maintain a low surface friction index for repeated use to obtain a good cleaning property of the polymerization toner. For example, a technology is known which improves the cleaning property of a polymerization toner by applying a solid lubricant such as zinc stearate to the surface of the image bearing member (for example, Nobuo Momotake, Akihisa Maruyama and Satoshi Shigesaki, Japan Hardcopy Fall Meeting, 24-27, 2001)
The technology concept for improving the durability by containing a filler in a resin layer forming the surface layer of an organic image bearing member is well known. For example, JOP 2007-79244 describes a technology in which silicon resin particulates (Example 2) or aluminum particulates (Examples 3 to 5) are contained in resin liquid application for the surface layer as a filler. JOP 2005-99688 describes a technology in which aluminum particulates (Examples 1 and 7 to 15), silica particulates (Example 2), titanium oxide particulates (Example 3), DLC and non-crystal carbon particulates (Example 4), fullerene particulates (Example 5), or colloidal silica (Example 6) is contained as a filler. JOP 2006-250989 describes a technology in which aluminum particulates (Examples 30, 31 and 46) are contained as a filler. JOP H08-234471 describes a technology in which particulates having silicon atoms having small particle diameter and silicon atoms having a large particle diameter are contained as a filler (Examples 1 to 4). JOP H08-314174 describes a technology in which two kinds of silica particulates having different specific gravities or two different kinds of complex metal oxide particulates are contained as a filler. JOP 2004-78113 describes a technology in which electroconductive particles (zinc oxide, titanium oxide, tin oxide, antimony oxide, indium oxide, bismuth oxide, indium oxide in which tin is doped, tin oxide in which antimony is doped, and zirconium oxide in which antimony is doped) are contained.
When a solid lubricant such as zinc stearate is externally supplied to a highly durable image bearing member in which radical polymerizable acryl cross-linked layers are accumulated (refer to JOP H11-288194 with regard to the external supply of a solid lubricant), a problem arises that the solid lubricant is not sufficiently accepted to the surface of the image bearing member. The surface of this kind of image bearing member is too smooth to observe concave-convex portions. Therefore, this problem is considered to stem from this surface smoothness of the image bearing member. By contrast, JOP 2007-79244 describes a technology of stably supplying a lubricant to the surface of an image bearing member by roughening the surface. To be specific, an image bearing member having a surface roughness (Rz: JIS 1994) of from 0.4 to 1.0 μm is advantageous and an addition of a filler to the surface layer is suitable to maintain a specific surface roughness thereof for such an image bearing member.
However, the rough surfaces can be various even when image bearing members have the same Rz value. For example, image bearing members having an extremely different concave and convex distance may have the same Rz value. The acceptability of an image bearing member for a solid lubricant may have order of precedence among image bearing members having the same Rz value. To improve the acceptability of an image bearing member for a solid lubricant, a specific condition other than the Rz value is required.
Furthermore, the addition of a filler to the surface layer involves the next problem. Aluminum particulates are used in Examples of JOP 2007-79244. Since the filler dispersability of the aluminum particulates in a liquid application is unstable, some device is required for the layer forming conditions. In another Example in which polymethyl silsesquioxane particulates are used, the acceptability of a solid lubricant on the surface of an image bearing member is not necessarily sufficient. This is considered to be because the surface of the image bearing member is significantly rough so that the image bearing member does not sufficiently bear the solid lubricant.
The liquid application for a cross-linked resin surface layer has a low viscosity because the liquid application is mainly formed of a monomer composition. However, since particulates containing silicon such as silica or silicone resin particulates are generally stably dispersed in the liquid application for a cross-linked resin surface layer, the particulates containing silicon have advantages among various kinds of fillers in terms of manufacturing. JOP 2005-99688 describes an example using particulates containing silicon. However, the acceptability of a solid lubricant on the surface of an image bearing member is not necessarily sufficient in this case. This is also considered to be because the surface of the image bearing member is significantly rough so that the image bearing member does not sufficiently bear the solid lubricant. Thus, addition of another technology is desirable.
In addition, JOP H08-248663 describes a technology in which inorganic particulates (hydrophobized silica) having an average particle diameter of from 0.05 to 0.5 μm are dispersed with a thickness of from 0.05 to 15 μm in the photosensitive layer having a surface roughness of from 0.1 to 0.5 μm formed on the electroconductive substrate having a surface roughness of from 0.01 to 2 μm.
This technology is to improve the durability of an image bearing member and prevent the definition decrease caused by attachment of contamination material such as corona products by hydrophobizing silica particles to be dispersed. In this technology, droplets are shed (i.e., contact angle is large) due to the hydrophobized inorganic particulates. However, the attachment of the corona product is not prevented so that the image flow is not prevented. JOP 2004-138643 describes a technology of using aluminum as a filler to avoid occurrence of image flow. However, as described above, filling aluminum in a cross-linked surface layer causes a manufacturing problem. Therefore, it is difficult to use aluminum as it is as a filler.
In an image forming apparatus in which a solid lubricant is externally supplied to the surface of the image bearing member, the acceptability of an image bearing member for a solid lubricant has an impact on the abrasion speed of the image bearing member, the cleaning property of toner and thereby on the image quality.